I have a vague concept for a game basically like Shadow of the Colossus. I remember playing it as a kid and it was my favorite game. I was really sad there was no multiplayer and they never released a sequel. I was thinking of making a game like it, a huge open world with giant titans roaming around that you can climb around on and slay. I want multiplayer as well and I think an in-depth crafting/customization system for armor/weapons would be cool. I was also contemplating some RPG elements like leveling, a skill tree, classes. So what do you think, does this vague concept sound like something you would play?

So, MMORPG of the Colossus? Sure, it's got potential to be interesting but it seems like all you have right now is a game you liked as a kid mixed with Today's Buzzwords for Commercial Success™. Unless you're already skilled at making such a complex game, I'd strongly suggest you start with something way more simplistic (maybe adding only one buzzword ) and seeing where that takes you.

But, as it is now, there's not much to say aside from what's already been said: vague idea is vague.

Do you plan on working on this yourself? What makes you think you could re-create a game like SoC let alone something much more than that?

Do you plan on refining the idea and giving/selling it to some developer? I can tell you, a project like this does require a huge team and commercial devs don't have a shortage on ideas and they have pretty qualified teams providing.

I have a vague concept for a game basically like Parappa The Rapper. It's like Parappa The Rapper, but there is multiplayer. Does this vague concept sound like something you would play?

Seriously though, it just sounds like you are sad that there wasn't a sequel to Shadow Of The Colossus so you want to make a sequel that includes multiplayer. This idea is neither vague, as the game already exists, nor is it YOUR idea.